Raised pavement markers have evolved over the past sixty years and serve to delineate areas such as traffic lanes on pavement surfaces. Most pavement markers include some form of reflector to improve motorists' nighttime visual perception. The benefits of raised pavement markers include improved wet reflectivity wherein the markers delineate areas when precipitation on pavement surfaces reflects light from vehicle headlights obscuring markings painted on the pavement. Another benefit of raised pavement markings is improved nighttime visibility over long distances. This contributes to lower motorist visual stress, thus lower fatigue, yielding improved safety for nighttime driving.
Raised pavement markers have been installed at the pavement surface using a variety of fixation means including metal pins, nails, screws or alternatively an adhesive means. Such markers are shown in U.S. Pat. No. 1,802,940 (Cornelius), U.S. Pat. No. 4,088,416 (Taylor), and U.S. Pat. No. 4,717,281 (Shepherd et al.). The use of metal poses a threat because dislodgement of such components may result in increased hazards for vehicles. Metal pins and screws, once dislodged, could find their way into vehicle tires or they could, as the markers themselves, become projectiles on roadways when acted upon by vehicular traffic, particularly at high speeds. One sure method of dislodgement for these types of markers can be from the action of snowplows.
Raised pavement markers have been devised that address the action of snowplows. Such raised, snowplowable pavement markers usually require boring or cutting a cavity within the pavement surface for fixation means. In general, raised snowplowable pavement markers appear as three broad types in their construction:
a) rigid, non-articulated, non-deformable units that withstand substantial impact forces of snowplow blades at high speeds as shown in U.S. Pat. No. 4,147,447 (Heenan et al.), U.S. Pat. No. 4,577,992 (Jefferies) and U.S. Pat. No. 4,634,310 (Clarke). Although somewhat simplistic in their construction, these units typically employ cast metal bases to withstand high impact forces imparted to them by snowplows. Portions of the markers are disposed above the pavement surface and deflect snowplow blades off the pavement surface to protect the reflector mounted on the marker. Retroreflectors mounted on these units also are typically rigid in their construction. These markers although useful, present several difficulties for roadway maintenance operations. Plowblades and the vehicles to which they are attached can suffer considerable fatigue and damage as a result of the plowblade striking the cast metal surfaces at high speeds. The cast metal ramps on this type of pavement marker deflect plowblades upwards off the pavement surface. This can leave snow and ice on the pavement surface in the area immediately about and beyond the marker impact point. The plowblades upon returning to the road surface also may damage the road surface upon impact and can damage or destroy pavement markings resulting in increased pavement maintenance costs. Snowplow operators have experienced nausea from prolonged plowing operations on roadways employing such markers as the impact energy and loud noise transmitted to the plow vehicle enhances operator stress levels. At highway speeds, plow impact with markers can occur as often as once a second. Plowing in snowstorm conditions, in traffic, at high speed for many hours is stressful enough without this additional persistent distraction.
b) articulated units that are mechanical or employ compressible components to allow marker sections that are raised above the pavement to move or retract to survive impacts from snowplows and vehicle tires. Mechanical units such as shown in U.S. Pat. No. 4,140,418 (Holley) and U.S. Pat. No. 4,848,958 (Sheldon) employ metal components that ultimately may deteriorate from the effects of salt and sand materials used on pavements during snow and icing conditions. Although these types of marker are unlikely to impede pavement plowing operations, they require substantial open cavities in pavement to operate.
The cavities are problematic because they are likely to trap debris. Frozen water within the cavities may impede proper function of the units and lead to their destruction by snowplows. U.S. Pat. No. 5,302,048 (Paulos et al.) discloses a pavement marker that can have several of its components replaced, however, the marker employs elaborate mechanical components to provide such, and the impact energy required to deflect the marker top section is believed to be essentially of the same magnitude as that experienced with the rigid, non-deformable units thus presenting many of the road maintenance concerns associated with the rigid units. Further, maintenance of the marker may be difficult.
c) resilient, deformable units, that employ cavities in the pavement to situate and anchor the markers. Most involve a body structure that contains a reflector--although U.S. Pat. No. 3,890,054 (O'Connor) and U.S. Pat. No. 4,815,818 (Thomas) disclose arrays of resilient, fingerlike projections that are reflectorized. Deformable, resilient body types are disclosed in U.S. Pat. No. 4,297,051 (Robinson), U.S. Pat. No. 4,659,248 (Flanagan) and U.S. Pat. No. 5,069,577 (Murphy). Flanagan discloses a unit that resides totally below the pavement surface and would likely survive the action of snowplows but like the mechanical units of Holley and Sheldon, a substantial pavement cavity is required that may fill with debris or water, rendering the retroreflector imperceivable to vehicular traffic. Robinson and Murphy and U.S. Pat. No. 3,850,536 (Kone) employ a cavity cut into the pavement that is intended to accommodate the upper portion of the marker body when deformed by vehicular traffic and snowplow blades. Kone desires air, trapped within the market cavity, to assist in proper operation of the marker. Robinson suggests that with a suitable grout, the internal cavity is airtight and assists the marker in returning to its normal shape and attitude following deformation. The markers are anchored in the cavities in the pavement by capturing a skirt and flange portion of the marker body using suitable grouts. As the side walls or skirt portion of the marker bodies are integrally molded with the flexible top portions, there is potential for side wall deformation upon snowplow impact. Robinson makes provisions to strengthen or stiffen this portion of the marker body to prevent damage to the device upon plow impact.
Most snowplowable pavement markers in use today are rigid, non-deformable cast metal base type markers containing rigid cube corner retroreflectors that are adhesively bonded (usually epoxy) to the metal base. Such markers, as previously described and cited, are installed by cutting or boring a suitable cavity in the pavement to accept the lower portion of the metal base and then fixating the units in the cavity with a suitable epoxy, cement or similar rigid grout. Because of road closure time limitations, these grouts are usually fast setting/rapid cure type materials. Aside from any intended cavities within the marker bodies themselves, the entire cavity cut in the pavement typically is filled with the grout and the marker body material leaving no voids within the pavement.
Given that retroreflectors on raised snowplowable pavement markers are exposed to traffic and studded tires, tire chains, snowplows, road chemicals, and weather, they do not last forever. Reasonable lifetimes are on the order of 2 to 3 years. Efficient replacement of the retroreflectors poses problems in that if a retroreflector is easily removed for replacement, it is likely to become dislodged on impact by vehicular traffic or snowplows. Few published documents disclose details on maintenance or refurbishing methods, or go on to address the inevitable requirements when pavement resurfacing is required.